RECENT PROGRESS IN INTRINSIC FIBEROPTIC CHEMICAL SENSING-II

Intrinsic fiber-optic chemical sensors, in which an optical fiber, or a section of fiber, plays a role in the transduction of chemical concentration information into optical information, have been reported in the scientific literature since 1946. This paper presents a very brief summary of early work in the field, and reviews the progress that has taken place in both passive (evanescent-field refractometric or spectroscopic) and active (chemically modified cores or cladding) sensing since 1989. Intrinsic fiber-optic refractometry continues to attract attention because of its simple geometry and its compatibility with a variety of hostile sensing environments. Evanescent-wave spectroscopic sensors have gained acceptance in some of the same fields as refractometric sensors, but are restricted to use in special environments because of stiff competition from plain 'end-coupled' spectroscopic approaches. Fluorimetry and surface-enhanced Raman spectroscopy (SERS) are two subfields in which direct evanescent spectroscopy remains strong. Coating-based sensors have made the most rapid progress. Recent reports include the development of hydrocarbon sensors based on refractive-index-changing polymer coatings, and numerous examples of coatings doped with fluorescent and absorptive sensor dyes. Core-based sensors, particularly those based on porous optical fibers, have undergone a great deal of development over the past few years. The first reports of intrinsic fiber-optic chemical sensor products involve these sensors. In general, intrinsic sensors have attracted increased attention over the past few years, primarily as a result of the interest in the sensing community at large in distributed and multiplexed sensing methodologies. These sensors offer the promise of true distributed measurement, in-line multiplexing, and continuous large-scale production.